Administration > Structure > Content types (admin/structure/types)
For each content type it is possible to enable or disable the PF link
via the "Show printer-friendly version link" checkbox. (default:
enabled)
It is also possible to enable or disable the PF link in individual
comments via the "Show printer-friendly version link in individual
comments" checkbox. (default: disabled)

Administration > Configuration > User interface > Printer, email and PDF versions (admin/config/user-interface/print)
This is where all the module-specific configuration options can be set.

- To modify the template of printer friendly pages, simply edit the
print.tpl.php or the css/print.css files.

- It is possible to set per-content-type and/or theme-specific templates
which are searched for in the following order:
1. print--[format]--node--[type].tpl.php in the theme directory
2. print--[format].tpl.php in the theme directory
3. print--node--[type].tpl.php in the theme directory
4. print.tpl.php in the theme directory
5. print.tpl.php in the module directory (supplied by the module)

The *_insert_link functions are available to content developers that prefer
to place the printer-friendly link in a custom location. It is advisable to
disable the regular Printer-friendly link so that it is not shown in both
locations.

Calling the function like this:

print_insert_link()

will return the HTML for a link pointing to a Printer-friendly version of
the current page.

It is also possible to specify the link to the page yourself:

print_insert_link("print/42")

will return the HTML pointing to the printer-friendly version of node 42.